Altimeter to mbar is being worked on - it will be implemented in the next software update.
The cutoff display occurs when the G5 is being drawn at too high a resolution for that aircraft's Panel.png, so it is drawing beyond the size of the "canvas". At https://afms.im/g5wiki under G5 acfConfig.cfg description, you can see how the config file is written. You can change the width and height of the G5 window setting g5_w_a/b for width and g5_h_a/b for height, and g5_x_a/b and g5_y_a/b for the coordinate of the bottom left corner of the display (full description at page linked).
For the C172, the installer should have installed "Cessna 172SP G5" into your Aircraft/X-Aviation folder with all the configuration already made and ready to fly.